CAN PRESIDENT REAGAN get away with suddenly deflating expectations about economic recovery that he himself had raised so high during the past two years, while stubbornly refusing to change course?

This question is hauntingly familiar to someone returning here from Britain after chronicling the first three years of Prime Minister Margaret Thatcher's similar experiment in survivial-of-the-fittest economics of tight money and budget cutting.

Thatcher's economic strategy, like Reagan's now, was long perceived to be failing as Britain plunged deeper into its worst recession in a half century, and Thatcher's approval rating in the polls plummeted to record lows. But Thatcher only redoubled her efforts to preach the gospel of Victorian economic virtue, warning Britons that "I'm afraid some things will get worse before they get better."

And, even before her successful prosecution of the war with Argentina over the Falkland Islands, she appeared to be succeeding. Despite only shadowy indications of a faltering improvement in the British economy, based mostly on an increase in industrial efficiency through layoffs and reforms forced by the severity of the recession, Thatcher and her Conservative Party had been making a marked, if somewhat mysterious, recovery in opinion polls.

Peculiarly British factors may account for some of this. Britons have long been inexplicably accepting of a steady economic decline relative to other western nations since World War II; the opposition Labor Party appears to be moving too far to the left for the tastes of British swing voters, and Thatcher's neo-Victorian image appears to appeal to many middle-class Britons disillusioned with the post-war welfare state nurtured with high taxes by both upper-class Conservatives and working-class Labor politicians.

But Thatcher's image of strong leadership and unswerving resolve, now strongly reinforced by the Falklands adventure, also appears to have given credibility to her insistence that she can make her economic policies work in the long run if given enough time. While Reagan now is asking voters who win of nuclear weapons developmell go to the polls in the fall congressional elections to give him another two years to show progress in ending American economic stagnation, Thatcher has told her voters she needs at least another five-year term, and maybe more, to turn the British economy around.

Reagan's new appeal to American people to have patience and endure pain while he tries to make his economic program work was filled with echoes of what Thatcher has been preaching for more than three years now. He virtually plagerized Thatcher's repeated exhortations to "resist the calls for easy options" that would mean a return to using "artificial means to have an artificial boom." Blaming "a generation" of government inflation of the economy for Britain's woes since she became prime minister -- just as Reagan blamed two decades of mostly Democratic government deficits for the still bigger budget holes he now finds himself in -- Thatcher has habitually warned that a necessary fundamental restructuring of the economy "cannot be painless."

Ever since Reagan's election, his supporters have fought off comparisons between his economic policies and Thatcher's because she appeared to be doing so badly. Now Reagan also has been forced to admit that his strategy has been painfully slow to produce any sign of beneficial results. He could only claim, as Thatcher has for months in Britain, that the recession has finally "bottomed out."

He has been left, like Thatcher before the Falklands war, with only rhetoric and symbolism as tools with which to maintain political credibility and create what they both describe as a "psychological" change in the country's economic expectations that they expect to lead, almost magically, to increased prosperity.

Thatcher, with some success, has summoned up images of Britain's imperial past and emphasized Victorian virtues of industry, charity, self-reliance and duty. She has been ridiculed by much of the British intelligensia for this and reviled by champions of the working class for trying to turn back the clock to the bad old days before the egalitarianism of the welfare state.

But she appears to have struck a chord among many members of Britain's belatedly growing middle class, who agree with her that drastic change is needed, while still waiting to she if can keep her promises to reduce taxes and energize the private economy.

White House and other opinion polls here show that many Americans -- especially the formerly Democratic voters who switched to Reagan in 1980 -- feel the same way about his presidency. If he can satisfy them for the moment with Thatcher-like rhetoric about the balanced budget amendment, his "new federalism" proposal and the virtue of self- sacrifice for the greater good, the polls show he may have another year in a less-patient America to make real progress in rescuing the economy. This is the apparent foundation for the current White House approach to the fall congressional elections.

Even before the British victory in the Falklands, Thatcher's aides believed she had weathered the worst storms over her economic policies and could still carry her party to victory in Britain's next national election in 1983 or 1984, with the firmness of her resolve in adversity their best campaign issue. They may now have an opportunity to see this judgment pretested by how Reagan's Republicans fare with a strikingly similar political strategy.